1. Field of the Invention
The present invention is broadly directed to the field of forming a uniform material layer on a moving belt and, more particularly, to the field of forming and ion-exchanging a uniform layer of mineral, ore or catalyst filter cake.
2. Discussion of the Prior Art
It is well known that various types of solid materials can be effectively processed by creating a material layer on a moving vacuum filter belt and then treating the material layer as it passes along on the filter belt. This technique is very common in the processing of minerals and is also well known in the field of producing catalysts. As an example, a phosphate ore is processed by depositing a slurry containing the phosphate-containing ore on a vacuum belt, to form a filter cake, and then ion-exchanging the filter cake by flowing an ion-exchanging liquid over and through the moving filter cake as it is carried along by the filter belt. The ion-exchange step reduces undesirable ions which are carried by the filter cake.
The basic process of ion-exchange of a catalyst filter cake to enhance its hydrocarbon conversion effectiveness by reducing ionic poisons is described, for example, in U.S. Pat. Nos. 4,048,284 and 3,943,233. These patents disclose the formation of a catalyst filter cake on a horizontal vacuum filter belt by depositing a slurry of catalyst particles on the filter belt which form the filter cake and then subjecting the catalyst filter cake to an ion-exchange process with an ion-exchange solution to produce a filter cake which can be further processed into a hydrocarbon cracking catalyst.
The liquid slurry containing the mineral, ore or catalyst in finely divided solid form is layered on the moving vacuum filter belt to a thickness of 1/4 to 2" by means of a weir box positioned above the moving filter belt. The ion exchanging liquid which flows onto the thin filter cake is deposited by another weir box in order to avoid breaking up or forming crevices on the surface of the body of the filter cake. One or a plurality of ion-exchange stages are typically used, each having one or more weir boxes positioned above the horizontal filter belt carrying the filter cake. After ion-exchange, the filter cake is then typically washed by flowing a water or other washing substance over and through the ion-exchanged filter cake by means of another weir box positioned above the horizontal filter belt, after which the filter cake is recovered and processed further.
In order to maximize the ion exchange conditions existing between the filter cake and ion exchanging liquid, an efficient contact between the exchange liquid and filter cake transported on the filter belt is required, which requires a filter cake of uniform thickness. Any break-through of the exchange liquid because of a non-uniform distribution of the thickness of the filter cake causes a higher than normal retention of replaced ion. This causes difficulty since the undesirable ions are retained by the filter cake.
The outlet of a conventional slurry weir box has the saw tooth profile illustrated in FIG. 2. It has been found that this type of weir box outlet construction does not always produce a uniform filter cake thickness. It is believed that this is due to surges in, or uneven liquid level of, the slurry in the weir box.
Because of the non-uniformity produced in the thickness of the filter cake, changes in the ionic content of a wet filter cake can be observed when all other process variables are seemingly held constant. The variability of the ionic content is accordingly attributed to transient non-uniform distribution of a slurry of, for example, minerals, ore or catalyst on the filter belt.